A brake pad for a disc brake comprises a backing plate having a back surface and a front surface essentially parallel to each other. A layer of friction material is adhered to the front surface. The friction material is applied to the front surface of the backing plate, at a predefined engaging portion which usually takes up most of the area available on said front surface. This engaging portion of the backing plate is further surrounded, without interruption, by a free peripheral zone, i.e. a zone intended not to be covered with the friction material. The peripheral zone, free of friction material, is necessary mainly for working requirements. The width of the free zone is usually kept to the bare minimum, as it is disadvantageous, mainly for reasons of weight, that the total area of the front surface of the backing plate is greater than the effective area for the braking effect. The minimum width of this free zone, usually measured along the longitudinal flanks of the backing plate, can vary from some tenths of millimeter to some millimeters.
During operation of a brake assembly, i.e. during braking, the brake pad is brought against the disc along a direction substantially perpendicular to the rotating surface of the disc. During braking the layer of friction material with which the brake pad is provided is exposed to forces that are mainly tangential with respect to the circular motion of the disc. Said tangential forces develop in the plane of the brake pad parallel to the disc and tend to remove the friction material by tearing it away from the back plate. During braking, in addition to these tangential forces, there are also other forces that are directed in several directions. Particularly, there are forces that are perpendicular to the surface of the brake pad and are caused by vibrations. Vibrations are generated during action of the brake pad on the disc, for instance as a consequence of possible alignment errors of the parts in relative motion. Furthermore, part of the kinetic energy of the disc turns into heat during braking. The heat thus generated usually causes an increase in the temperature of the materials of which the brake pad is made. The increase in temperature may cause weakening of the bonds that hold the friction material on the backing plate, with a consequent risk that said material becomes detached. The greater the braking action to be exerted by the brake assembly onto the rotating disc is, the stronger the forces acting on the friction material during braking are. When such forces overcome the counter-forces holding the fiction material adhering to the back plate, detachment and destruction of the friction material or of part thereof occur, thus causing jeopardy to or loss of the braking effect.
One of the problems to be solved in the making of a brake pad is therefore that of how to prevent detachment of the friction material during use. Some solutions are known in the art in order to promote adhesion of the friction material to the backing plate.
A first solution to the aforesaid problem provides to make protrusions on the front surface of the backing plate intended for receiving the friction material. During the manufacturing of the brake pad, the friction material is deposited onto the surface of the backing plate. When the layer of friction material is hardened, the protrusions remain embedded in said layer. The protrusions penetrating into the layer of friction material therefore contribute to prevent detachment of the friction material from the backing plate when shear forces, even strong ones, are applied. According to prior art, these protrusions can be obtained by deposition of material, which can be for instance welded to the front surface of the backing plate. U.S. Pat. No. 4,991,697 describes an example of this kind of solution. According to another known solution, the protrusions can also be obtained by material removal from the surface of the backing plate, for instance by means of movable blades. Cutting of the surface of the backing plate by means of movable blades causes partial removal of the material, thus creating corresponding recesses. The material removed by the movable blades is lifted from the surface of the backing plate, but it is not completely detached. The removed material therefore remains lifted, thus creating protrusions or asperities. Each protrusion is associated to a corresponding engagement recess, which, however, is not the main purpose of working with movable blades. The protrusions and the corresponding engagement recesses thus obtained can be distributed in a random manner, as described for instance in EP 1 484 524 A1, or they can be distributed in a uniform manner, as described for instance in U.S. Pat. No. 6,431,331 B1. Another solution to the aforementioned problem provides to make, on the surface of the backing plate, solely engagement recesses, without making any asperities, thus leaving the surface of the backing plate substantially smooth. This solution is described in US 2011/0220441 A1 and provides to make, on the front surface of the backing plate, a regular matrix of engagement recesses having a square cross-section. The engagement recesses are preliminarily made on the band of material from which the backing plate will be subsequently cut away by means of known blanking methods. According to the teaching of this document, the matrix of engagement recesses has a regular shape, which determines a regular distribution of the engagement recesses. Still according to the teaching of this document, it is further provided that the side walls of the engagement recess extend perpendicularly to the surface of the backing plate on which said engagement recesses are made and that the base of the recesses is parallel to the front surface of the backing plate.
The problem of how to prevent the friction material from becoming detached, however, is not effectively solved by known solutions. Indeed, these solutions generically deal with promoting the adhesion of the friction material to any flat surface. However, it has been demonstrated in practice that in a brake pad the problem of the detachment of friction material does not uniformly affect the surface of the brake pad. The results obtained by means of known solutions, moreover, are not constant upon varying of some parameters of the brake pad. The parameters that can influence the behavior of the known solutions are, in particular, the kind of steel of which the backing plate is made, the kind of friction material, the size and shape of the backing plate and the size of the brake disc. The kind of material of which the backing plate is made and the thickness thereof determine for instance the capacity of dissipating heath. The shape of the backing plate and the shape of the disc determine the direction and the intensity of the shear forces applied to the friction material. The granulometry, the density and the binder used in the mixture of the friction material make the methods described above more or less effective depending on the variation of these parameters.
A strong need is therefore felt in the field to solve the problem of how to promote adhesion of the friction material to the backing plate, by avoiding occurrences of detachment, upon variation of these parameters, thus allowing to obtain better and more uniform results upon variation of the kind of brake pad which is manufactured.
The main object of the invention is to solve the problem mentioned above, by providing a method of obtaining a backing plate of a brake pad that improves adhesion of the friction material to the brake pad.
A further object of the invention is to provide a method of the aforesaid kind that is easy and cost-effective to perform and is therefore suitable for being used on a large scale in industry.